LOGSTASH-952: Allow configurable drop isolated events on repeat failure.

Currently Logstash will keep retrying indexing an event on failure,
but some events will never succeed (esp. due to ES index type errors).

This patch provides an option (drop_on_repeated_failure) to allow
some data loss when an event fails to be indexed repeatedly. It does
so by bisecting the bulk request into smaller requests as they fail;
when a single event fails to index 3 times in a row, it will be
dropped and the event will be logged as a string.

See [LOGSTASH-952](https://logstash.jira.com/browse/LOGSTASH-952)